This invention relates to a Loran receiver which receives pulse signals transmitted from a master and a slave station and measures the time difference in arrival between both pulse signals.
In a Loran receiver, the time difference in arrival between pulse signals from a pair of master and slave stations is measured to determine one position line on which the Loran receiver is positioned, and the time difference between pulse signals from another pair of master and slave stations is measured to determine another position line, and then the intersection of the two position lines is read out from a Loran chart, thereby locating the position of the Loran receiver. For such localization, it is necessary to measure two time differences between pulse signals from at least two pairs of master and slave stations, as referred to above. In the prior art, measured time differences between pulse signals from each of a plurality of pairs of master and slave stations, which are obtained in a Loran receiver, are sequentially displayed on one display while being manually switched by an operator, and the two most suitable ones of the measured time differences for positioning purposes are selected and displayed alternately with each other, and based on the displayed values, a Loran chart is read out. This conventional method involves such manual selection of the suitable measured time differences, and hence is inevitably complicated in operation.
In Loran C or D, a group of pulses are repeatedly transmitted, and the presence or absence of an inverted carrier phase between adjacent ones of the pulses in the pulse group is encoded to indicate from which station the pulses are transmitted. A receiver of Loran C or D, decides the phase code of the received pulse group to select a desired Loran pulse. In some cases, however, a faulty phase code may be produced by noises or like components, resulting in a pseudo-Loran signal being detected. To avoid this, it is the practice in the prior art to detect a desired Loran signal a plurality of times in succession, produce a measurement pulse synchronized with the Loran signal and employ it for measurement of the time difference. Hence, in the prior art, upon detection of a desired phase code, it is assumed to be a correct signal, on the basis of which sample pulses of a Loran repetitive period are developed, and a plurality of successive sample pulses are each checked for the detection of a phase code. If it is not detected, the same operation is carried out based on a phase code obtained after the abovesaid plurality of sample pulses, and this operation is repeated until phase codes are detected respectively in coincidence with a plurality of sample pulses. Such a conventional method has the drawback that the detection of a correct Loran signal requires as long a time as 20 to 60 seconds or more in dependence on the SN ratio of the received signal.
An object of this invention is to provide a Loran receiver which permits a marked simplification of the operation for selecting desired measured time differences.
Another object of this invention is to provide a Loran receiver which ensures detection of a correct Loran signal within a short time without seizing a pseudo-signal.
Still another object of this invention is to provide a Loran receiver which is very simple in operation for selective display of measured time differences and ensures the detection of a correct Loran signal in a short time.